In a typical commodity lumber mill, boards are cut from a log and then transferred to a machine center for edging or other processing. The processed boards may then be transferred to other machine centers for further processing. Many machine centers are designed to accept boards at regular intervals, one at a time, from a lugged transfer deck. Lugged transfer decks are also used to singulate boards for sorting and grading.
In its simplest form, a lugged transfer deck comprises a series of parallel longitudinally oriented chains which all move at the same speed. Lugs project upwardly at spaced intervals along the chains. The lugs on the chains are aligned in transverse rows across the transfer deck. The boards lie transversely on top of the chains. As the chains move, the rows of lugs push the boards along the transfer deck.
The boards are singulated as they are loaded onto the lugged transfer deck. That is, only one board is placed transversely on the lugged transfer deck between adjacent rows of lugs. The lugged transfer deck is preferably fully loaded with one and only one board carried by every row of lugs. Singulating the boards enables functions such as scanning, positioning, sorting, turning etc., to be accomplished for each board individually. Keeping the lugged transfer deck fully loaded maximizes the number of boards which are processed by a machine center which is fed by the lugged transfer deck.
Boards are often produced at somewhat random times by the process preceding a lugged transfer deck. To ensure a steady supply of boards to the lugged transfer deck, boards are typically accumulated in a single layer on a smooth transfer deck just before the lugged transfer deck.
A smooth transfer deck comprises a number of smooth longitudinally oriented spaced apart chains which move together along the transfer deck. Boards are laid transversely on the chains. As the chains move the boards are carried along the smooth transfer deck. If the boards are prevented by an obstruction from moving along the transfer deck, the smooth chains continue to slide along under the boards until the obstruction is removed and the forward progress of the boards resumes.
The lugged transfer deck normally operates continuously and uniformly to suit the downstream process requirements, with every lug designed to receive a piece of lumber. Typical lugged deck speeds are approximately 60/80 lugs per minute, or 200 feet per minute. The smooth transfer deck generally moves more slowly than the lugged transfer deck, typically at approximately 100 feet per minute.
A lug loader is provided between the smooth transfer deck and the lugged transfer deck. The purpose of the lug loader is to take boards from a closely spaced accumulation of boards on the smooth transfer deck, and transfer them,one at a time, onto the lugged transfer deck. The lug loader must place exactly one board in front of each successive row of lugs on the lugged transfer deck.
Prior art lug loaders typically provide a stop at the end of the smooth transfer deck. Boards accumulate behind the stop. This ensures a consistent supply of lumber for loading by the lug loader. When a board is to be loaded it must be accelerated to a speed close to that of the chains on the lugged transfer deck. This is accomplished by an accelerating section. In prior art lug loaders the accelerating section typically comprises short chains and friction wheels, spaced apart and located across the smooth transfer deck, positioned parallel with and projecting slightly higher than the smooth transfer deck chain runners, and located just past the stop. The chains and wheels of the accelerating section are positioned to contact the underside of a board as it moves past the stop. The accelerating section is typically driven mechanically from the lugged transfer deck.
In such prior art lug loaders, the loading cycle begins when a board is located against the stop (as determined by an electronic switch of a known design), and the row of lugs to which the board is to be loaded has reached a predetermined location. The stop is then lowered to allow the leading board to pass the stop and enter the accelerating section. In the accelerating section, the friction between the bottom of the board and the moving chains and wheels of the accelerating section accelerates the board and propels the board onto the moving lug chain of the lugged transfer deck.
When the leading board is accelerated a gap is created between the leading board and the board immediately behind it on the smooth transfer deck. An electronic switch detects this gap. The stop is inserted into the gap to prevent the following board from moving onto the acceleration section. While the leading board is being accelerated, the smooth transfer deck carries the following board up to the stop where it is ready to be loaded when the next row of lugs moves into position. When the next row of lugs is in position the process is repeated.
Because prior art lug loaders generally rely upon friction on the bottom of a board to draw the boards into their acceleration sections and to accelerate boards in their acceleration sections, such lug loaders use a stop which projects upwardly from below the level of the smooth transfer deck. When the stop is in its raised position it slightly lifts the leading edge of the leading board so that it does not contact the moving parts of the acceleration section. This prevents the leading edges of the boards from being damaged by contact with the moving parts of the acceleration section while they are held back by the stop.
The stops in prior art lug loaders typically comprise a series of parallel pivoting arms having vertical stop surfaces. The pivoting arms are connected to an actuating mechanism. The vertical stop surfaces are normally located along each side of the smooth transfer deck chains and before the lugged deck tail sprockets. In its raised position the stop projects above the smooth transfer deck by an amount approximately equal to the thickness of one of the boards being loaded. The lugs passing around the lugged deck tail sprocket, have clearance to pass the stationary lumber positioned behind the stop.
The speed of the lug loader is often the limiting factor in determining how much lumber can be sent along a lugged transfer deck to a machine center or a sorting center. Machine center processing speeds have increased substantially in recent times. The speeds of the lugged transfer decks which carry boards to such machine centers have also increased. Today, speeds of approximately 110 to 130 lugs per minute are desired. These lug speeds are equal to chain speeds in the range of approximately 200 to 350 feet per minute.
At such high speeds, the accelerating section in prior art lug loaders is generally incapable of accelerating a board to the speed of the lugged transfer deck chains in the time interval between successive lugs. This is because in prior art lug loaders the forces used to accelerate the boards to be loaded are almost exclusively derived from friction between the lower surface of the boards to be loaded and moving chains and/or wheels in the acceleration section. The magnitude of such forces is limited by the coefficient of friction between the board and such moving frictional surfaces and the weight of the board.
If the force required to accelerate the board to the required speed is greater than the frictional force developed between the board and the frictional surfaces in the accelerating region then the board will slip excessively relative to the friction surfaces. This excessive slippage is especially severe on frozen lumber. The coefficient of friction between frozen lumber and a friction surface is typically much lower than the coefficient of friction between non-frozen lumber and the same frictional surface.
Another problem with prior art lug loaders is that, in some circumstances, boards can jump over the lower movable stops used in such lug loaders. As noted above, the stop mechanisms used in prior art lug loaders comprise a number of vertical stop surfaces which project upwardly from below the smooth transfer deck. Such stop mechanisms are open at the top. This geometry occasionally allows boards which do not sit flat on the smooth transfer deck to be pushed over the top of the stop. This can occur, for example for boards which are bowed or which have waney lower faces (i.e. faces which are not flat due to being cut from the outside of the log). When the friction force developed between the underside of the board and the smooth chain moving against it, forces a non-vertical face of a board against the vertical surface of the stop the face of the board can be forced to ride up the face of the stop and then over the stop. When this occurs, the operation of loading lumber into lugs is interrupted, and lost production ensues.
A third problem with standard prior art lug loaders is that the ratio of the speed of the smooth transfer deck to that of the lugged transfer deck is generally either fixed or time consuming to change. The relative speeds chosen are a compromise to accommodate the widest boards which will be loaded. Each time a board is loaded, the boards behind it on the smooth transfer deck must be moved ahead by the width of the board before the next lug on the lugged transfer deck is in position to be loaded. Therefore, the smooth transfer deck should operate faster when wider boards are being loaded than when narrower boards are being loaded.
When there is a mixture of batches of wide and narrow boards on the smooth transfer deck, the narrower pieces spend more time against the stop than they would if the system were optimized for narrow boards. Therefore, narrow boards experience more chain slippage than necessary on their bottom surfaces while they are held back by the stop waiting to be loaded. Excessive sliding of the smooth transfer deck chains beneath the boards, can mark the boards unnecessarily.